What is vitamin D?
Vitamin D is a fat-soluble secosteroid hormone synthesised in skin cells upon exposure to UVB radiation. It exists in two supplemental forms: D2 (ergocalciferol, plant-derived) and D3 (cholecalciferol, animal-derived). D3 is more effective at raising and maintaining serum 25-hydroxyvitamin D levels, which is the primary marker used to assess vitamin D status.
How it works
Vitamin D is converted in the liver and kidneys to its active form, calcitriol, which functions as a hormone. It regulates calcium and phosphorus absorption in the gut, supports bone mineralisation, modulates immune cell function, and influences gene expression in muscle tissue. Vitamin D receptors are present in virtually every cell type in the body.
What the research says
- Vitamin D deficiency (below 50 nmol/L) is associated with reduced muscle strength, increased injury risk, and impaired immune function
- Supplementation in deficient athletes improved muscle strength and power output (Wyon et al., 2014)
- 4000 IU daily for 6 months significantly improved VO2 max in deficient athletes compared to placebo
- Australian data shows 23–43% of adults have insufficient vitamin D levels despite high sun exposure
Effective dose
1000–4000 IU of vitamin D3 daily with a fat-containing meal for optimal absorption. Those with confirmed deficiency may require higher doses under medical supervision. Serum 25(OH)D levels of 75–100 nmol/L are generally considered optimal for athletic performance. A blood test is the only reliable way to assess current status.
What to check on the label
Ensure the product specifies D3 (cholecalciferol) rather than D2. Most quality supplements use D3. Doses below 400 IU are unlikely to move serum levels meaningfully. Many multivitamins contain vitamin D at insufficient doses — a standalone D3 supplement gives better control over intake.
Safety
Safe at doses up to 4000 IU per day for most adults. Toxicity is possible but rare — typically only with prolonged supplementation above 10,000 IU daily. Vitamin D is fat-soluble and accumulates in tissue, so more is not always better without confirmed deficiency. Blood testing before and during supplementation is recommended.